CS217968B2 - Reactor for oxydation of the alkylaromate in the liquid phase under utilization of tha gs containing the oxygen - Google Patents

Abstract

A multi-stage reactor for the oxidation of alkyl aromatics, e.g. a mixture of p-xylene and methyl p-toluate in a liquid phase reaction mixture with oxygen-containing gases, e.g. air, under elevated pressure and at an elevated temperature in the presence of an oxidation catalyst is in the form of an elongated closed tank with a multiplicity of neighboring reaction chambers arranged successively from one end to the other end of the tank for containing the liquid reaction mixture at predetermined levels in each chamber. The reactor is provided with an oxidizing gas feed system for introducing an oxidizing gas into each chamber, feed means for introducing at least one alkyl aromatic reactant and an oxidation catalyst into at least one of the reaction chambers, a vapor-collecting conduit means in communication with each of said chambers for removing reaction gas from each of said chambers, and a discharge means for removing the oxidized product from the reactor tank. An internally disposed cooling conduit system containing a coolant for removing the heat of reaction is provided and includes a group of cooling conduits for each of the reaction chambers. These groups have horizontally disposed cooling conduits through which a cooling medium flows.

Description

The invention relates to a reactor for the oxidation of alkylaromates, in particular a mixture of p-xylene and methyl p-toluylic acid by means of oxygen-containing gases, which consists of a cylindrical vessel placed horizontally or vertically, separated by partitions or bottoms into adjacent reaction chambers. These chambers are equipped with bundles of horizontally mounted cross-flow cooling tubes and an oxidant gas supply, and part of the chambers are equipped with p-xylene feed, catalyst and p-toluylic acid methyl ester, vapor vapor line and otxidate removal.

The invention relates to a reactor for the oxidation of alkylaromates, in particular. a mixture of p-xylene and toluylic acid methyl ester, in the liquid phase and using oxygen-containing gases.

Known reactor types for these oxidations consist of cylindrical vessels about 18 to 30 m in height with vertically mounted longitudinally circumferential cooling tubes to dissipate the heat of reaction by means of a cooling medium (see, for example, U.S. Patent No. 3,065,061).

Cascades of stirred reactors without tubular internals are known for other methods of oxidizing p-xylene to terephthalic acid. Here, cooling is achieved directly by evaporating the solvent used.

The oxidation of the mixtures of p-xylene and methyl p-toluyl ester takes place in several successive stages, with the oxidizing gas flowing through the reaction space from the bottom up. Two superimposed oxidant gas conduits to one oxidation stage are also known, one oxidizer being used for each stage and the reaction product passes through the individual stages continuously. In this embodiment, as many as possible reaction steps are desired, as this allows optimum reaction conditions to be set.

It is also known that the reaction proceeds in the last steps. slower than in the first stage. This effect results in less use of oxygen-containing gas, such as air, as the oxidant gas, when penetrating the last stage of oxygen than in the first stage. In order to compensate for this effect, in practice, the starting product is also added to the last reaction steps.

The liquid column to which the oxidizing gas must flow is determined by the height of the oxidizers. The gas flow creates a pressure difference between the liquid surface and the air introducing system at the bottom of the oxidizers, which must be overcome by operating pressure, for example by means of a compressor.

The height of the oxidizers is determined by the design of the cooling surface. The required size of the cooling surface is obtained by making long vertical pipes that are at the top. and curved down, lead through the side wall of the oxidizers. These cooling tubes exit outside in the attached steam or condenser chambers. The number of cooling tubes is limited by the number of openings in the oxidizer walls due to the strength of the device. If oxidizers are to be designed for larger quantities to be processed, only a solution in which the cooling tubes are extended is possible to increase the cooling surface. In addition to the increased cost of the equipment, this leads to the larger oxidizers already envisaged, or to an increase in their cross-sections, which are so large as to cause difficulties in transporting the parts from the production site to the installation site.

Further, a structure is known in which the vertical cooling tubes in the reactor are connected in a header in the reactor head. This arrangement also leads to considerable construction heights at high investment costs.

If the oxidation is carried out in a plurality of stages, in the currently known oxidizers, each stage is provided with a measuring and control technology connected by a conduit, a safety device, a base of the like, resulting in relatively few stages being planned in terms of cost. The longitudinally circulating cooling tubes in the prior art oxidizer have relatively poor heat transfer. In known constructions, it is therefore necessary to build relatively high oxidizers with respect to the required cooling surface. This results in a poor utilization of the volume of the device and therefore the yield per unit volume is negligible.

The amount of air added to the relatively small cross-section of the oxidizers causes a pan-like consistency in the contents of the reactor, which can easily cause it to be foamed.

High overlapping of the bottom leads to an increase in the amount of oxidizing gas already used inside the liquid.

The introduction of the oxidizing gas is arranged such that the gas is injected from below into the oxidizer and oxygen is consumed in the oxidizers as it flows upwards. The oxidizing gas in the oxidizer must be heated before the reaction occurs. In the current constructions, the stirrers are not installed due to the necessity of very long shafts and the associated investment. costs and technical difficulties.

Due to the large construction heights of the present oxidizers, the reaction product has to remain unnecessarily long at the temperatures required for oxidation, leading to decomposition reactions, thereby reducing the yield.

In the currently known oxidizers, almost the entire upper half of the reaction space must be installed only in order to achieve the cooling surface required to dissipate the reaction heat.

The larger part of the cooling surface is far from the air supply. This leads to the fact that the path of heat transport from the place where the heat of reaction is generated to the places where the heat is dissipated is very long. This results in local overheating of the product, which also leads to product losses.

Experimental measurements have shown that when using air as the oxidant gas for mixtures of p-xylene and methyl esters of p-toluylic acid, oxygen has been consumed or consumed to the extent that the oxygen concentration in the off-gas is far away after passing through a column of reaction mixture of 3 meters or less. below the explosion limit.

Heat propagation calculations have shown that the achieved heat transfer values at given ratios according to the prior art considered can be substantially improved by other cooling tube arrangements.

Furthermore, it is known that oxidation of mixtures of p-xylene and methyl esters of p-toluylo217968 in the liquid phase and in the presence of heavy metal-containing oxidation catalysts can result in the formation of solids such as terephthalic acid on the walls of the oxidizer and particularly on the cooling tubes. However, the process management and surface quality of the cooling tubes have been so greatly improved in recent years that no deposits of solids have been observed on the horizontal or inclined cooling tubes.

It is an object of the present invention to overcome the aforementioned disadvantages of the prior art apparatus.

It is therefore an object of the present invention to overcome the following disadvantages: high pressure drop, long heat transfer path. It is further an object of the invention to increase the yield per unit of space compared to that achieved with prior art devices and to reduce investment costs, space requirements and energy consumption, and to eliminate the risk of foaming. In addition, the number of stages of the oxidizer of the present invention is to be increased from the current assumption of one stage per oxidizer to two or more stages per oxidizer.

The nature of a liquid-phase alkylaromate oxidation reactor using oxygen-containing gases, in particular mixtures of p-xylene and methyl p-toluyl ester, at elevated pressure and temperature and the presence of an oxidation catalyst consisting of a cylindrical vessel, oxidation gas supply system, p-xylene, p-toluylic acid catalyst and methyl ester, vapors vapor line, oxidate removal system, and an internal cooling tubing system comprising a heat sink to remove the heat of reaction according to the invention is characterized in that the cylindrical vessel comprises a plurality of adjacent reaction chambers wherein all reaction chambers are provided with bundles of horizontally mounted cross-flow cooling tubes and an oxidant gas inlet, and part of the reaction chambers are provided with p-xylene, catalyst and p-toluylic acid methyl ester feeds, vapors and from water oxidate.

It will be apparent to those skilled in the art from the following particularly preferred embodiments that a number of advantages can be achieved with the apparatus of the present invention in the catalytic oxidation of p-xylene and methyl p-toluyl ester in a liquid phase using an oxygen-containing gas and pressure and temperature, which either prior art devices could not be realized or could only be realized with considerable technical difficulties.

In addition, using the apparatus according to the invention, liquid-phase catalytic oxidation of known other alkyl carbonates is possible with oxygen-containing gases at elevated pressure and temperature, for example oxidation of m-xylene and its oxidation intermediates to isophthalic acid, oxidation of toluene to benzoic acid, oxidation of p- xylene or p-tolualdehyde to terephthalic acid and the like.

In a particularly preferred embodiment of the present invention, the device consists of a horizontally lying cylindrical vessel in which the adjacent reaction chambers are separated from one another by partitions or closed bottoms and connected by a system of connected vapor vapor lines.

The cylindrical container according to the invention can also be arranged obliquely.

Various embodiments of preferred embodiments of the device according to the present invention are shown in the drawings, wherein Figures 1 and 2 show a horizontally lying oxidizer with side-by-side reaction chambers and separate baffles and an auxiliary device, Figs. 5 and 6, a vertically arranged oxidizer with the above-mentioned reaction chambers separated from each other and auxiliary devices, fig. 7 · a vertically arranged oxidizer with the above-mentioned reaction chambers separated from each other and superimposed, 8 is a detail of a coolant pipe system with coolant recycling.

1 and 2 show an oxidizer in the form of a horizontally lying cylindrical vessel 1 composed of a plurality of adjacent reaction chambers separated by baffles 5. These reaction chambers are provided with a tubular cooling system 2, an oxidation gas supply 3, through the vapor line 7, possibly the return line 8 and the compressor 19 (FIG. 2), the completely or partially reacted gas is discharged, or is fed back through the feed line 4 to the first reaction chambers. Feeds 12, 13 and 14 of the starting materials serve to feed p-xylene, methyl p-toluyl ester and catalyst. The heat exchanger 20 serves to preheat the oxidizing gas.

Figures 3 and 4 show an oxidizer 1 with reaction chambers separated by partitions 5 and one bottom 6, with different pressures being achieved by means of the oxidate pump 9. Other reference numerals have the same meaning as in Figures 1 and 2.

In the reaction chamber separated by the bottom 6, a pressure is maintained such that not yet completely reacted oxidizing gas can be passed through the return line 8 without the formation of bubbles into the first reaction chamber.

According to the embodiment of the apparatus of Figures 5 and 6, the chambers are arranged one above the other in the oxidizer 1, wherein the bottoms 6 are separated from each other and comprise tubular cooling systems 2, oxidation gas inlet 3 and return 4 and oxidate outlet 15. Through the return line 8, the partially reacted gas is passed from the lower reaction chamber to the upper chamber. At different pressures, the oxidate is pumped from the upper and lower reaction chambers by means of pumps 217968 and 9 to allow the off-gas to be passed. Other reference numerals have the same meanings as the preceding illustrations.

According to the embodiment of the apparatus of Fig. 7, the reaction gases from one reaction chamber are led to the second stack 16, which are so large that the liquid can flow in a countercurrent flow into the next reaction chamber. In the upper reaction chamber, stacked chimneys 17 are formed from above. In this case, the chimneys of the upper reaction chamber are provided with side openings or bores to ensure the passage of the oxidizing gas.

However, it is so well possible that the chimneys in the upper reaction chamber remain open. In this case, no pump is required. 9. The other reference numerals have the same meaning as in Figures 1 to 6.

At the same pressure in all chambers, the return line 8 is provided with a compressor 19 and possibly a cooling 18 as shown. FIG.

For the oxidation gas inlet 3 and the gas return line 4, the free area of the horizontally positioned oxidizer of FIGS. 1, 2, 3 and 4 is substantially greater than that of the vertically constructed oxidizer of FIGS. 5, 6 and 7. air injection can be achieved with a horizontal oxidizer by increasing the length of the reactor.

The available liquid height of a horizontal reactor is given by its cross section,. whereas in a vertical reactor, a certain height can be set at each stage. In the horizontal reactor, any return flow combinations for both the oxidate and the oxidizing gas are possible. When the reaction components are transported from one reaction chamber to the other by means of a pump 9, fluid control valves are connected with the pump in series (see Figures 3, 4, 6 and 7).

The residence time of the reaction components in the liquid phase or in the slurry is adjusted for the individual reaction chambers by means of the filling state control elements (see Figs. 3, 4, 5, 6 and 7).

In tubular cooling system. 2, natural cooling evaporative cooling is normally possible if surface temperatures are such that crystallization of the product is not possible. In deciding whether to choose natural circulation or forced circulation during condensate evaporation, the economic calculations are decisive. Cooling by circulating liquid or gas without evaporation is less energy-efficient due to the occurring double resistance in the heat transfer. It is also possible to use circulating liquid systems in which the heated liquid evaporates in the evaporator chamber.

A typical example of natural circulation. 8 is a cross-sectional view perpendicular to the longitudinal axis of the horizontally arranged cylindrical vessel 1 of the present invention with a cooling tube system 2, as well as a coolant circuit (not shown) associated therewith. To the right of the vertical dashed line (see Fig. 8), which extends through the center of the cross-section of the horizontally cylindrical container 1, is shown a longitudinal section in a vertically arranged cylindrical container 1 with a cooling tube system 2 bounded by an upper and lower angle.

In both cases, the refrigerant circuit system operates on the thermosiphon principle, i.e., steam from the evaporator in the form of a tube bundle rises to the condensate separator illustrated (no reference numeral). The separated condensate is recirculated to the cooling pipe system 2 again.

Compared to the prior art, the following advantages are achieved by the construction according to the invention: The required reaction spaces are considerably smaller, the space and space required for the construction according to the invention being about 2.0 to 50% of the values required so far. For the prior art, very long cooling tubes (e.g. 18 meters) were required. The lengths of the cooling tubes in the oxidizers of the invention are about 5 to 6 meters. Large steam chambers fall off, making welds more controllable and accessible. The crack in the tube in the previous constructions has resulted in a substantial production failure, since the entire oxidizer had to be shut down for repair.

Compared to the prior art, a plurality of tubular cooling systems 2, including cooling tubes, are incorporated in the structures of the present invention, so that failure of one cooling system does not necessarily lead to production interruptions. A defective tubular cooling system can be shut down and repaired at the appropriate opportunity. Due to the small diameters of the oxidizers of the present invention, there is no problem to date in transporting parts of the apparatus to a destination.

An improved configuration of a cooling pipe system that is laterally bypassed with oxidizing gas and reaction. components, the cooling surfaces are · 20 to 50% smaller. Savings depend on the selected cooling system. In a natural circulation system with normal heat transfer resistance, the cooling surface savings are within the lower range of the range.

The following Table I shows a comparison between prior art oxidizers and between the oxidizers of the present invention at the same space yield and the same cooling intensity. The required cooling surface and the oxidizer volume as well as the air passage area are compared.

1 7 9

Table I horizontal oxidizer with transversely rounded tubes grade

3 conventional oxidizer method

3 number

cooling surface 2 600 2 300 1 900 1400 1 200 1 200 (m ² ) volume (m ³ ) 399 404 410.5 138 113 120 flat 19 Dec 18 18 25.9 21.6 2 ^! 2.2

air flow rate (m ² )

The values in Table I are applicable to oxidation in a three-step process to produce about 140,000 t of dimethyl terephthalate per year, oxidizing the mixture of p-xylene and methyl p-toluyl ester with air in the liquid phase and in the presence of an oxidation catalyst containing heavy metals. The process is carried out at elevated pressure and elevated temperature, predominantly in a mixture composed of p-toluylic acid and monomethyl terephthalate, followed by esterification with methanol and separation of the resulting mixture of ethers.

In the conventional process, Table I shows a separate arrangement of three consecutive oxidizers.

The second part of the table applies to the horizontal oxidizer of the present invention, which is constructed approximately according to FIGS. 1 to 4.

From the values in the table, it is apparent that only about 56% of the cooling surface is required for the horizontal reactor apparatus with cross-flow cooling tubes compared to the conventional arrangement of three consecutive conventional oxidizers.

According to the values given in Table I, the volume required for the apparatus of the present invention is only about 30% of the volume required in the conventional arrangement of three consecutive oxidizers.

The air passage area of the device according to the present invention is, according to the data in Table I, substantially greater than that of a conventional oxidizer arrangement.

In view of these figures, the investment costs for the apparatus of the present invention are substantially lower compared to the prior art arrangement at a higher power level. Significant savings are also achieved by eliminating the connected pipe lines, measuring and control devices, safety fittings and construction costs, compared to the three-stage design with three consecutive individual oxidizers.

Using the oxidizers of the present invention, a plurality of reaction stages can be realized without substantially increasing the cost. Optimum process control and uniform dwell times also result in higher process selectivity. This leads to energy savings and improved yield.

In the case of a horizontal reactor, the spatial distance between the oxidation gas introduction point and the cooling surface is reduced to a fraction of the necessary values to date. As a result, the heat of reaction can be removed immediately, which, together with the smaller volume of the oxidizers of the present invention, allows for a gentler treatment of the product, higher chemical yields and higher yield per unit of time per unit of space. For example, from a technical point of view, it is possible to install agitators for better dispersion of air or oxidizing gas, circulation pumps for more efficient mixing of the reaction components also in the upper chambers of the standing oxidizers according to FIGS. 5, 6 and 7. This leads to greater throughput, uniform residence times and better use of oxygen.

By returning the already reacted and cooled vapors to another reaction chamber, the vortex is intensified, the cooling effect is increased, the oxygen distribution is improved, and the oxygen concentration is reduced.

When recycling the reaction gas, it is possible to work with a variable concentration of oxygen, or to increase turbulence, which leads to greater throughput and optimization of oxidation conditions.

Also achievable preheating the oxidizing gas increases the reaction rate of the oxidation.

Compression investment can be reduced by a proportion that corresponds to the saved column of liquid.

The usual residual oxygen content of about 4% in the last stage can be completely eliminated with the oxidizers of the present invention, in particular with the upright structures of Figures 6 and 7.

1 to 8 show preferred embodiments of the structure according to the present invention. Other forms of the device which are not shown are routinely deductible to those skilled in the art from the disclosure.

The following is an exemplary embodiment for the oxidation of p-xylene and methyl p-toluyl ester in the apparatus of the present invention.

Example 1

The continuous oxidation of the mixture of 16,208 kg / h of p-xylene and 23,878 kg / h of methyl p-toluyl ester is carried out in a horizontally placed reactor made of stainless steel. The reactor has a plurality of adjacent reaction chambers, which are sequentially flowed through the reaction mixture, with bundles of horizontally installed cooling tubes as a cooling system. Further, the reactor comprises a system for introducing oxidizing gas in all reaction chambers, as well as feeds, vapor lines and a system for the oxidate circuit in a portion of the reaction chambers. The reactor has a volume of 370 m ³ and a cooling surface of 3800 m ^2, with 122 kg / h of catalyst solution containing 30 g / l Co ²⁺ and 2.4 g / l Mn ²⁺ and 51 584 kg / h being introduced into it. air. The process is carried out at a temperature in the range of 155-165 ° C and a pressure of about 70 bar. 38 382 kg / h of oxidate and 53 350 kg / h of off-gas are obtained, the oxidate having the following composition: 2 to 5% by weight of high-boiling fractions, 12 to 18% by weight of terephthalic acid, 20 to 28% by weight of monomethyl terephthalate, 10 to 15% by weight of dimethyl terephthalate, dimethyl isophthalate and 17-23% by weight of p-toluic acid, 16-22% by weight of methyl p-toluyl ester, 3-6% by weight of methyl benzoate, 1-2% by weight of p-xylene and the remainder being low-boiling .

Claims (12)

A reactor for the oxidation of alkylaromates in a liquid phase using oxygen-containing gases, in particular mixtures of p-xylene and methyl p-toluyl ester, at elevated pressure and temperature and in the presence of an oxidation catalyst consisting of a cylindrical vessel, oxidizing gas supply system; inlets for p-xylene, catalyst and methyl ester of p-toluylic acid, vapor line, oxidate removal system and an internal tubular cooling system comprising a coolant for dissipating the heat of reaction, characterized in that the cylindrical vessel (1) comprises a plurality of adjacent reaction chambers, all reaction chambers are equipped with bundles of horizontally mounted cross-flow cooling tubes (2) and oxidation gas inlet (3) and part of the reaction chambers are equipped with p-xylene inlets (12, 13, 14), catalyst and p-toluylic acid methyl ester, via the vapor line (7) and discharge (15) oxidate. Reactor according to Claim 1, characterized in that circulation pumps (10) are arranged in the individual reaction chambers. Reactor according to Claims 1 and 2, characterized in that the reaction chambers are provided with reaction mixture pumps (9) controlled by level controllers. 4. The reactor as claimed in claim 1, characterized in that the reaction chambers are equipped with stirrers (11), optionally designed as air diffusers. 5. A reactor as claimed in any one of claims 1 to 4, comprising a heat exchanger (20). 6. The reactor as claimed in claim 1, characterized in that it comprises a compressor (19), a return line (8) and a vapor return line (4) from the last reaction chamber. 7. The reactor according to claim 1, characterized in that the horizontally lying cylindrical vessel (1) comprises side-by-side reaction chambers separated from one another by the baffles (5), wherein a common vapor space is formed in the vessel. Reactor according to Claims 1 to 6, characterized in that in the horizontally lying cylindrical vessel (1) the reaction chambers or part of the reaction chambers are separated by means of closed days (6). 9. A reactor as claimed in claim 1, wherein the cylindrical vessel (1) is arranged vertically and is divided into closed reaction chambers (6) into reaction chambers. Reactor according to Claim 9, characterized in that the reaction chambers are connected to one another via an external vapor line (7). 11. A reactor as claimed in claim 9, characterized in that it is provided with vertical, open and liquid-level reaction liquid bounded by stacks (16). 12. A reactor as claimed in claim 9, characterized in that the upper closed chimneys (17) are provided with side openings for introducing vapor vapors into the first upper reaction chamber separated by a bottom (6) from the underlying reaction chambers.